Low-aberration high-speed-compatible optical delay lines and methods thereof

ABSTRACT

This disclosure describes an example architecture for providing a delay line for optical techniques. The delay line architecture includes a focusing element that has a focal axis disposed parallel to its length. The line of symmetry provided by the focal axis obviates path-length-dependent aberrations caused by the off-axis beam translations. The systems described herein also provide varying geometries of movable mirrors, including a galvanometer mirror and a rotating polygonal mirror. The systems and methods described herein also provide techniques for generating and detecting coherent Raman spectra using a picosecond probe pulse.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority, and benefit under 35 U.S.C. § 119(e),to U.S. Provisional Patent Application Nos. 62/862,598, filed 17 Jun.2019, and 63/004,777, filed 3 Apr. 2020, the entire contents of whichare hereby incorporated by reference as if fully set forth below.

FIELD OF THE DISCLOSURE

Embodiments of the present disclosure relate generally to systems andmethods for providing an optical delay line and, more particularly, tosystems and methods for providing an optical delay line for use inoptical techniques such as coherent anti-Stokes Raman scattering.

BACKGROUND

Many linear and non-linear optical methods such as interferometry andspectroscopy, such as pump-probe spectroscopy, require optical delay ofone light path with respect to the other. Delay line performancerequirements vary with application but are particularly demanding whendealing with broad spectral ranges and ultrafast pulses; especially ifrapid scanning is necessary. The requirement of matched dispersion forthe delayed and reference beam, encountered when working with very shortpulses, generally requires a dispersionless free-space delay line. Insuch arrangements, optical paths are ordinarily mechanically modulated.

Mechanical path modulation is easily accomplished at low speed withlinear motion of a reflecting mirror along the optical axis. Sucharrangements introduce no significant aberrations, but they are limitedin scan rate by inertial constrains on mirror acceleration. Repetitionrate can be increased by replacing linear motion with oscillatory orrotational motion of a reflecting surface. These configurations,however, also suffer from various drawbacks, such as low duty factor,significant static wavefront distortion from uncompensated reflectionsat curved surfaces, difficulty in fabrication, or off-axis beamdisplacement.

These drawbacks are significant concerns for many optical techniquesthat require low aberrations and high scan rate. For example, inapplications such as broadband Fourier-transform coherent anti-StokesRaman scattering (FT-CARS), rapidly repeating, dispersion-free pulsedelay on the order of several ps is desired. What is needed, therefore,are systems and methods that resolve the problems associated with lowscan rate and high wavefront aberration. Ideally, such systems andmethods can be implemented to improve fingerprint coherent Ramanmicroscopy and other optical techniques.

SUMMARY

Embodiments of the present disclosure address these concerns as well asother needs that will become apparent upon reading the description belowin conjunction with the drawings. Briefly described, embodiments of thepresent disclosure relate generally to systems and methods for providingan optical delay line and, more particularly, to systems and methods forproviding an optical delay line for use in optical techniques such ascoherent anti-Stokes Raman scattering and other coherent Raman methods.

An exemplary embodiment of the present invention provides a method foradjusting an optical path length. The method can include directing afirst light field to a delay module. The delay module can include amovable mirror, a focusing optical element having a focal axis parallelto its length, and a return mirror. The method can include delaying thefirst light field via the delay module to create a delayed light field.The movable mirror can be configured to receive the first light fieldand reflect the first light field to the focusing optical element. Thefocusing optical element can be configured to receive the first lightfield reflected from the movable mirror and return the first light fieldto the return mirror. The return mirror can be configured to receive thefirst light field reflected from the focusing optical element andreflect the first light field back to the focusing optical element.

In any of the embodiments described herein, the movable mirror can berotatable upon an axis. The movable mirror can be positioned such thatthe movable mirror intersects the focal axis of the focusing opticalelement. The return mirror can be positioned such that the return mirrorintersects the focal axis of the focusing optical element. The firstlight field can approach the delay module along a line which is a linearcombination of the axis of the movable mirror and the focal axis of thefocusing optical element. The method can further include rotating themovable mirror upon the axis to direct the first light field toward thefocusing optical element.

In any of the embodiments described herein, the focusing optical elementcan be a cylindrical mirror.

In any of the embodiments described herein, the movable mirror can be aplanar galvanometer mirror rotatable upon an axis. The method canfurther include rotating the movable mirror upon the axis to direct thefirst light field toward the focusing optical element.

In any of the embodiments described herein, the movable mirror can be apolygonal mirror rotatable around a rotational axis. The method canfurther include rotating the movable mirror upon the rotational axis todirect the first light field toward the focusing optical element.

In any of the embodiments described herein, the method can furtherinclude repeating the directing and the delaying steps a plurality oftimes to extend a delay range and/or reduce aberrations.

In any of the embodiments described herein, the method can furtherinclude directing an input light field across a beam splitter such thatthe input light field is split into the first light field directed tothe delay module and a second light field directed to an object ofinterest. The method can further include combining the second lightfield with the delayed light field on a detector after the second lightfield interacts with the object of interest.

In any of the embodiments described herein, the method can furtherinclude calculating a position of the object of interest with respect tothe beam splitter based on optical interference between the delayedlight field and the second light field after the second light fieldinteracts with the object of interest.

In any of the embodiments described herein, the method can furtherinclude calculating a complex refractive index of the object of interestbased on optical interference between the delayed light field and thesecond light field after the second light field interacts with theobject of interest.

In any of the embodiments described herein, the method can furtherinclude directing an input light field across a beam splitter such thatthe input light field is split into the first light field and a secondlight field. The method can further include combining the second lightfield and the delayed light field to create combined light fields. Themethod can further include directing the combined light fields across anobject of interest thereby creating one or more new light fields withnew spectral frequency content. The method can further include directingthe one or more new light fields onto one or more detectors aftertransmission through or scattering from the object of interest. Themethod can further include determining a frequency-dependent phaseand/or amplitude of the one or more new light fields based on a delaybetween the delayed light field and the second light field.

In any of the embodiments described herein, the first light field andthe second light field can be optical pulses with spectral content ofgreater than 200 wavenumbers. The one or more new light fields can bedetected at higher optical frequencies than the delayed light field andthe second light field; or the one or more new light fields can bedetected at lower optical frequencies than the delayed light field andthe second light field.

In any of the embodiments described herein, the method can furtherinclude combining a fourth light field containing the same frequencycomponents as the one or more new light fields. The method can furtherinclude constituting a local oscillator with any of the light fieldsprevious to detection.

In any of the embodiments described herein, the method can furtherinclude combining a third light field with the combined light fields,the third light field being an optical pulse with spectral content ofless than 30 wavenumbers and being fixed in time with the second lightfield. The method can further include directing the combined lightfields with the third light field across an object of interest to createthe one or more new light fields. The one or more new light fields canbe detected at higher optical frequencies than the third light field; orthe one or more new light fields can be detected at lower opticalfrequencies than the third light field.

In any of the embodiments described herein, the method can furtherinclude detecting, at the one or more detectors, the one or more newlight fields along their primary polarization direction.

In any of the embodiments described herein, the method can furtherinclude detecting the one or more new light fields at the one or moredetectors. The one or more new light fields can be separately andsimultaneously detected at +45 degrees and at −45 degrees with respectto their primary polarization direction.

In any of the embodiments described herein, the method can furtherinclude directing an original light field across a first beam splittersuch that the original light field is split into the first light fieldand a second light field. The method can further include directing thesecond light field across an object of interest thereby creating a thirdlight field having new spectral frequency components compared to thesecond light field. The method can further include collecting the thirdlight field after the second light field scatters from the object ofinterest and/or after the second light field transmits through theobject of interest. The method can further include combining the thirdlight field with a portion of the delayed light field having the samefrequency components as the third light field in a second beam splitter,thereby creating combined light fields. The method can further includedirecting the combined light fields onto one or more detectors. Themethod can further include determining a frequency-dependent phaseand/or amplitude of the combined light fields based on a delay betweenthe delayed light field and the second light field.

In any of the embodiments described herein, the method can furtherinclude directing, prior to combining the third light field with theportion of the delayed light field, the delayed light field into amaterial to generate the portion of the delayed light field having thesame frequency components as the third light field.

In any of the embodiments described herein, the original light field cancontain the same frequencies as the third light field. The first beamsplitter can be dichroic, thereby separating a frequency of the firstlight field from a frequency of the original light field that are commonto new frequency components of the third light field.

In any of the embodiments described herein, the original light field canbe an optical pulse with spectral content of greater than 200wavenumbers. The third light field can be detected at opticalfrequencies higher than those in the second light field; or third lightfield can be detected at optical frequencies lower than those in thesecond light field.

In any of the embodiments described herein, the original light field canbe a combination of a first original light field and a second originallight field. The first original light field and the second originallight field can be combined co-linearly and coincidentally in time. Thefirst original light field can be an optical pulse with spectral contentof greater than 200 wavenumbers. The second original light field can bean optical pulse with spectral content of less than 30 wavenumbers. Thethird light field can be detected at optical frequencies higher thanthose in the second light field; or the third light field can bedetected at optical frequencies lower than those in the second lightfield.

In any of the embodiments described herein, the combined light fieldscan be detected along their primary polarization direction.

In any of the embodiments described herein, the combined light fieldscan be separately and simultaneously detected at +45 degrees and at −45degrees with respect to their primary polarization direction.

In any of the embodiments described herein, the method can furtherinclude directing an original light field across an object of interest,thereby creating a second light field with new spectral frequencycontent compared to the original light field. The method can furtherinclude collecting the second light field after it scatters from theobject of interest or after it transmits through the object of interest.The method can further include directing the second light field across afirst beam splitter such that the second light field is split into thefirst light field and a third light field. The method can furtherinclude combining the delayed light field and the third light field in asecond beam splitter to create combined light fields. The method canfurther include directing the combined light fields onto one or moredetectors. The method can further include determining afrequency-dependent amplitude of the combined light fields based on adelay between the delayed light field and the third light field.

In any of the embodiments described herein, the original light field canbe an optical pulse with spectral content of greater than 200wavenumbers. The first and third light fields can be detected at opticalfrequencies higher than those in the original light field; or the firstand third light fields can be detected at optical frequencies lower thanthose in the original light field.

In any of the embodiments described herein, the original light field canbe a combination of a first original light field and a second originallight field. The first original light field and the second originallight field can be combined co-linearly and coincidentally in time. Thefirst original light field can be an optical pulse with spectral contentof greater than 200 wavenumbers. The second original light field can bean optical pulse with spectral content of less than 30 wavenumbers. Thefirst and third light fields can be detected at optical frequencieshigher than those in the original light field; or the first and thirdlight fields can be detected at optical frequencies lower than those inthe first light field.

In any of the embodiments described herein, the first light field andthe third light field can be detected along their primary polarizationdirection.

In any of the embodiments described herein, the first light field andthe third light field can be separately and simultaneously detected at+45 degrees and at −45 degrees with respect to their primarypolarization direction.

Another exemplary embodiment of the present invention provides a systemfor providing an optical delay to a light field. The system can includea light field source configured to provide a first light field. Thesystem can include a delay module. The delay module can include amovable mirror, a focusing optical element having a focal axis parallelto its length, a return mirror. The movable mirror can be configured toreceive the first light field and reflect the first light field to thefocusing optical element. The focusing optical element can be configuredto receive the first light field reflected from the movable mirror andreturn the first light field to the return mirror. The return mirror canbe configured to receive the first light field reflected from thefocusing optical element and reflect the first light field back to thefocusing optical element.

In any of the embodiments described herein, the movable mirror can berotatable upon an axis. The movable mirror can be positioned such thatthe movable mirror intersects the focal axis of the focusing opticalelement. The return mirror can be positioned such that the return mirrorintersects the focal axis of the focusing optical element. The firstlight field can approach the delay module along a line which is a linearcombination of the axis of the movable mirror and the focal axis of thefocusing optical element.

In any of the embodiments described herein, the focusing optical elementcan be a cylindrical mirror.

In any of the embodiments described herein, the movable mirror can be aplanar galvanometer mirror rotatable upon an axis.

In any of the embodiments described herein, the movable mirror can be apolygonal mirror rotatable around a rotational axis.

In any of the embodiments described herein, the system can have a scanrate of greater than 1.0 kHz.

In any of the embodiments described herein, the system can have a scanrate of greater than 40.0 kHz.

Another exemplary embodiment of the present invention provides a method.The method can include directing an input light field across a beamsplitter such that the input light field is split into a first lightfield and a second light field. The method can include delaying thefirst light field with respect to the second light field, therebycreating a delayed light field. The method can include combining a thirdlight field with the second light field and the delayed light field tocreate combined light fields. The third light field can be an opticalpulse with spectral content of less than 30 wavenumbers and can be fixedin time with the second light field. The method can include directingthe combined light fields across an object of interest, thereby creatingone or more new light fields with spectral frequency content. The methodcan include directing the one or more new light fields onto one or moredetectors after transmission through or scattering from the object ofinterest. The method can include determining a frequency-dependent phaseand/or amplitude of the one or more new light fields based on a delaybetween the delayed light field and the second light field.

In any of the embodiments described herein, the first light field andthe second light field can be optical pulses with spectral content ofgreater than 200 wavenumbers. The one or more new light fields can bedetected at higher optical frequencies than the third light field; orthe one or more new light fields can be detected at lower opticalfrequencies than the third light field.

In any of the embodiments described herein, the method can furtherinclude detecting, at the one or more detectors, the one or more newlight fields along their primary polarization direction.

In any of the embodiments described herein, the method can furtherinclude detecting the one or more new light fields at the one or moredetectors. The one or more new light fields can be separately andsimultaneously detected at +45 degrees and at −45 degrees with respectto their primary polarization direction.

In any of the embodiments described herein, the step of delaying thefirst light field with respect to the second light field can includedirecting the first light field to a delay module. The delay module caninclude a movable mirror, a focusing optical element having a focal axisparallel to its length, and a return mirror. The movable mirror can beconfigured to receive the first light field and reflect the first lightfield to the focusing optical element. The focusing optical element canbe configured to receive the first light field reflected from themovable mirror and return the first light field to the return mirror.The return mirror can be configured to receive the first light fieldreflected from the focusing optical element and reflect the first lightfield back to the focusing optical element.

In any of the embodiments described herein, the movable mirror can berotatable upon an axis. The movable mirror can be positioned such thatthe movable mirror intersects the focal axis of the focusing opticalelement. The return mirror can be positioned such that the return mirrorintersects the focal axis of the focusing optical element. The firstlight field can approach the delay module along a line which is a linearcombination of the axis of the movable mirror and the focal axis of thefocusing optical element. The method can further include rotating themovable mirror upon the axis to direct the first light field toward thefocusing optical element.

In any of the embodiments described herein, the focusing optical elementcan be a cylindrical mirror.

In any of the embodiments described herein, the movable mirror can be aplanar galvanometer mirror rotatable upon an axis. The method canfurther include rotating the movable mirror upon the axis to direct thefirst light field toward the focusing optical element.

In any of the embodiments described herein, the movable mirror can be apolygonal mirror rotatable around a rotational axis. The method canfurther include rotating the movable mirror upon the rotational axis todirect the first light field toward the focusing optical element.

In any of the embodiments described herein, the method can furtherinclude redirecting the first light field to the delay module aplurality of times to extend a delay range and/or reduce aberrations.

Another exemplary embodiment of the present invention provides a method.The method can include directing a first light field across an object ofinterest. The method can further include directing a delayed light fieldacross the object of interest. The method can further include directinga picosecond probe light field across the object of interest. The methodcan further include creating a signal field with the first light field,the delayed light field, and the picosecond probe light field after thefirst light field, the delayed light field, and the picosecond probelight field interact with the object of interest. The method can furtherinclude analyzing the signal field with a broadband coherent anti-StokesRaman scattering technique after applying delay on the delayed lightfield.

In any of the embodiments described herein, the method can furtherinclude creating the delayed light field by: passing a second lightfield through a beam splitter to create a third light field; anddirecting the third light field to a movable mirror, to a cylindricalmirror having a focal axis parallel to a length of the cylindricalmirror, and to a return mirror.

In any of the embodiments described herein, the method can furtherinclude dispersing the signal field before applying it onto asingle-element or multi-element detector. The method can further includevarying timing of the delayed light field. The method can furtherinclude recovering signal oscillations based on a variation of thedelayed light field.

These and other aspects of the present invention are described in theDetailed Description below and the accompanying figures. Other aspectsand features of embodiments of the present invention will becomeapparent to those of ordinary skill in the art upon reviewing thefollowing description of specific, exemplary embodiments of the presentinvention in concert with the figures. While features of the presentinvention may be discussed relative to certain embodiments and figures,all embodiments of the present invention can include one or more of thefeatures discussed herein. Further, while one or more embodiments may bediscussed as having certain advantageous features, one or more of suchfeatures may also be used with the various embodiments of the inventiondiscussed herein. In similar fashion, while exemplary embodiments may bediscussed below as device, system, or method embodiments, it is to beunderstood that such exemplary embodiments can be implemented in variousdevices, systems, and methods of the present invention.

BRIEF DESCRIPTION OF THE FIGURES

Reference will now be made to the accompanying figures and diagrams,which are not necessarily drawn to scale, and wherein:

FIGS. 1A-1C depict an example layout for a reduced-aberration delay lineusing a planar galvanometer, according to aspects of the presentdisclosure;

FIGS. 1D-1H depict an example layout for a reduced-aberration delay lineusing a polygonal mirror, according to aspects of the presentdisclosure;

FIG. 2 depicts wavefront simulations for example delay linearchitectures;

FIG. 3 depicts the results of testing example delay line architectures;

FIGS. 4A and 4B are graphs of signal generation for prior art broadbandFourier-transform coherent anti-Stokes Raman scattering (FT-CARS)techniques;

FIG. 5 shows exemplary components of a dispersed Fourier Transformbroadband coherent anti-Stokes Raman scattering (dFT-BCARS) system,according to aspects of the present disclosure;

FIGS. 6A and 6B are schematic descriptions of a dFT-BCARS signalgeneration scheme, according to aspects of the present disclosure;

FIG. 7 depicts the results of testing a non-dispersed FT-BCARS and adispersed FT-BCARS system;

FIGS. 8A-8D are schematics for implementing a delay module, according toaspects of the present disclosure; and

FIGS. 9A and 9B are schematics for implementing fingerprint coherentRaman microscopy, according to aspects of the present disclosure.

DETAILED DESCRIPTION

Although certain embodiments of the disclosure are explained in detail,it is to be understood that other embodiments are contemplated.Accordingly, it is not intended that the disclosure is limited in itsscope to the details of construction and arrangement of components setforth in the following description or illustrated in the drawings. Otherembodiments of the disclosure are capable of being practiced or carriedout in various ways. Also, in describing the embodiments, specificterminology will be resorted to for the sake of clarity. It is intendedthat each term contemplates its broadest meaning as understood by thoseskilled in the art and includes all technical equivalents which operatein a similar manner to accomplish a similar purpose.

It should also be noted that, as used in the specification and theappended claims, the singular forms “a,” “an,” and “the” include pluralreferences unless the context clearly dictates otherwise. References toa composition containing “a” constituent is intended to include otherconstituents in addition to the one named.

Ranges may be expressed herein as from “about” or “approximately” or“substantially” one particular value and/or to “about” or“approximately” or “substantially” another particular value. When such arange is expressed, other exemplary embodiments include from the oneparticular value and/or to the other particular value.

Herein, the use of terms such as “having,” “has,” “including,” or“includes” are open-ended and are intended to have the same meaning asterms such as “comprising” or “comprises” and not preclude the presenceof other structure, material, or acts. Similarly, though the use ofterms such as “can” or “may” are intended to be open-ended and toreflect that structure, material, or acts are not necessary, the failureto use such terms is not intended to reflect that structure, material,or acts are essential. To the extent that structure, material, or actsare presently considered to be essential, they are identified as such.

It is also to be understood that the mention of one or more method stepsdoes not preclude the presence of additional method steps or interveningmethod steps between those steps expressly identified. Moreover,although the term “step” may be used herein to connote different aspectsof methods employed, the term should not be interpreted as implying anyparticular order among or between various steps herein disclosed unlessand except when the order of individual steps is explicitly required.

The components described hereinafter as making up various elements ofthe disclosure are intended to be illustrative and not restrictive. Manysuitable components that would perform the same or similar functions asthe components described herein are intended to be embraced within thescope of the disclosure. Such other components not described herein caninclude, but are not limited to, for example, similar components thatare developed after development of the presently disclosed subjectmatter. Additionally, the components described herein may apply to anyother component within the disclosure. Merely discussing a feature orcomponent in relation to one embodiment does not preclude the feature orcomponent from being used or associated with another embodiment.

To facilitate an understanding of the principles and features of thedisclosure, various illustrative embodiments are explained below. Inparticular, the presently disclosed subject matter is described in thecontext of systems and methods for providing an optical delay line thatcan be used in optical techniques such as interferometry, spectroscopy,and/or coherent anti-Stokes Raman scattering. The present disclosure,however, is not so limited and can be applicable in outer contexts. Forexample, some examples of the present disclosure may improve otheroptical techniques that require one reference light field to be delayedwith respect to another light field. Accordingly, when the presentdisclosure is described in the context of systems and methods forproviding an optical delay line that can be used in optical techniquessuch as interferometry, spectroscopy, and/or coherent anti-Stokes Ramanscattering, it will be understood that other embodiments can take theplace of those referred to.

As described above, various optical techniques such as interferometryand pump-probe spectroscopies require optical delay of one light pathwith respect to the other. Prior art designs for creating a delay lineincluded a series of mirrors, including a first mirror, a sphericalmirror, and a return mirror. An input light field was directed at thefirst mirror such that the light field would proceed from the firstmirror, to the spherical mirror, to the return mirror, and back. Thefirst mirror was oscillated to redirect the light field to a differentpoint on the spherical mirror, thereby changing the path length of thelight field and, thereby, creating a delay in the line. An issue withthese prior art designs was that the design suffered from off-axis beamdisplacement. The spherical mirror used in the designs had a singlefocal point. As the angle of the input light field changed (i.e., bymoving the first mirror), the beam of the input field hit the sphericalmirror farther from the center of the mirror. Wavefront aberrationsincreased as the input field traveled farther from the center of themirror, resulting in loss of interferometric contrast at longer pathdelays. FIG. 3 is illustrative of the 10-fold better spectral resolutionand signal-to-noise ratio obtained in coherent anti-Stokes Ramanscattering implementation of the new delay line compared to the priorart.

Delay Line

In applications such as broadband Fourier-transform coherent anti-StokesRaman scattering (FT-CARS), rapidly repeating, dispersion-free pulsedelay on the order of several ps is required. To solve the issue ofwavefront aberrations found in the prior art, the present disclosuredescribes a dispersion-free optical delay line that provides very lowaberration over a delay range of at least 10 ps. The design compensatesfor delay-dependent off-axis beam displacements by directing them alonga line of symmetry introduced with a focusing element, which can includea cylindrical mirror. The disclosure also provides two additionalimplementations for providing the delay line: one based on agalvanometric mirror (i.e., planar) and one based on a rotatingpolygonal mirror.

FIGS. 1A-1H provide example layouts for reduced-aberration delay lines.FIGS. 1A-1C depict an example layout for a reduced-aberration delay linewhere optical path changes are induced by a change in mirror angle(θ_(g)) of a galvanometer. FIGS. 1A and 1B show the X-Z and Y-Zprojections of the delay line using a cylindrical mirror and agalvanometer (Cyl/Gal) design; FIG. 1C is a schematic of the Cyl/Galdesign. A collimated light field 102 from a source 104 can be sent onthe edge of a movable mirror 106 a (a planar galvanometer mirror in thisexample), at a distance h_(g) from its axis 107. A focusing element 108can be placed so that the surface of the movable mirror 106 a at θ_(g)=0contains the focusing element's 108 focal axis 112. The focusing element108 can, for example, be a cylindrical mirror having a focal axis 112parallel to the length of the cylindrical mirror. A return mirror 110can be placed at the focus of the cylindrical mirror (as shown in FIGS.1B and 1C), so that the line-focused beams reflected along thez-direction by the cylindrical mirror are returned along the opticalpath.

The projection in FIG. 1A shows that the beam can be deflected at anangle 2 θg from the galvanometer mirror (i.e., movable mirror 106 a). Atthe cylindrical mirror (or other focusing element 108), the beam isreflected along a path that is quasi-parallel with the z-axis, as longas θ_(g) is small. As θ_(g) becomes large, the light sent to thecylindrical mirror from the galvanometer will appear to originate awayfrom its focal axis 112, so it will not be reflected parallel to thez-axis and will not be returned along its original path. This shows upas a delay-dependent wavefront x-tilt in FIG. 2, right column. In orderto minimize this effect, the θ_(g) can be minimized for any given delayΔI by making ϕ_(g) as small as possible.

The Y-Z plane projection in FIG. 1B shows two beams representing lightpaths at different galvanometer mirror displacements. As it rotates, thegalvanometer mirror moves along the incoming light path as Δr_(Z)=h_(g)tan(θ_(g))/sin(ϕ_(g)) and reflects the light to a displacement-dependentpoint on the cylindrical mirror. As shown, the locus of those pointsforms a line that follows that symmetry line of the cylindrical mirrorso that no path-length-dependent aberrations are introduced by theoff-axis beam translations in this plane.

Light reflected from the cylindrical mirror is returned by a returnmirror 110 that can be tilted slightly away from a plane parallel withthe cylindrical mirror (α_(g)≠β_(g)) in order to send the light backalong its original path in the Y-Z plane. This means that the opticalpath between the cylindrical mirror and retro-reflecting mirror canchange slightly with path delay. Accordingly, the return mirror 110plane may not contain the focal axis 112 of the cylindrical mirror atall delays, and a slight, delay-dependent astigmatism (wavefrontx-curvature in FIG. 2) can be present. This effect can be minimized byoptimizing α_(g) to reduce the difference |α_(g)−β_(g)|, which requiresϕ_(g) to be close to π/2.

Optical delay generated by galvanometer rotation is essentially linearfor small angle (≈±5°) and is given as:

$\begin{matrix}{{\Delta{l\left( \theta_{g} \right)}} = {2h{\frac{\tan\left( \theta_{g} \right)}{\sin\left( \phi_{g} \right)}\left\lbrack {1 + \frac{{\sin\left( \gamma_{g} \right)} + {\sin\left( {\alpha_{g} - \beta_{g} - \gamma_{g}} \right)}}{{\cos\left( \alpha_{g} \right)}\left( {1 + {{\sin\left( {2\phi_{g}} \right)}/{\sin\left( \alpha_{g} \right)}}} \right.}} \right\rbrack}}} & {{Equation}1}\end{matrix}$

where γ_(g)=2θ_(g)+α_(g)−π/2.

The two sources of aberrations can be mutually minimized withappropriate limits on ϕ_(g) that depend primarily on the focusingelement 108 curvature, the beam offset h_(g), and beam diameter. Theselimits on ϕ_(g) are largely relaxed with the polygonal mirror design. Tothis end, another aspect of the present disclosure provides a polygonalmovable mirror 106 b, wherein the movable mirror 106 b includes aplurality of facets 118 with which to deflect the incoming light field102.

FIGS. 1D-1H depict an example layout for a reduced-aberration delay linewhere optical path changes are induced by rotating a polygonal movablemirror 106 b (referred to herein as a “Cyl/Poly” design) around arotational axis 116. FIG. 1D shows the Cyl/Poly design, with directionof incoming light; FIG. 1E shows the path incoming light traces acrossthe mirror facets 118 of the polygonal mirror during scanner rotationaround the rotational axis 116; FIGS. 1F and 1G depict angulardeflection and translational displacement of the beam at discrete θ_(p)values in the X-Y and Y-Z planes, respectively; and FIG. 1H is aschematic of the Cyl/Poly design. Differently shaded paths reflectdistinct mirror rotations. A cylindrical mirror can be placed so thatthe loci of intersections between the incoming light field 102 and themovable mirror 106 b surface form a line coincident with the cylindricalmirror focal axis 112. As with the galvanometer (e.g., Cyl/Gal) design,a return mirror 110 can be placed at the cylindrical mirror focus, butdisplaced away from the polygonal movable mirror 106 b (as shown inFIGS. 1G and 1H) so that the line-focused beams reflected along thez-direction by the cylindrical mirror are returned along the opticalpath. Here, the light deflected by the polygonal movable mirror 106 balways originates at the cylindrical mirror focal axis 112, and noaberrations arise from the rotating movable mirror 106 b.

FIG. 1E shows the path traced by the intersecting light as the movablemirror 106 b rotates around the rotational axis 116. The positiondifferential in the plane of the facet 118 is given by R_(p)[1−cos(Θ_(p))] where R_(p) is the distance between the mirror rotational axis116 and the axis of light impinging on the polygonal movable mirror 106b. Accordingly, the differential path length given by intersection ofthe facet 118 and the light beam along the z-axis is given byΔh=R_(p)[1−cos (θ_(p))]tan (ϕ_(p)). Optical delay in the polygonalmovable mirror 106 b design is approximately parabolic in θ_(p), andgiven by:

$\begin{matrix}{{\Delta{l\left( \theta_{p} \right)}} = {{R_{p}\left\lbrack {1 - {\cos\left( \theta_{p} \right)}} \right\rbrack}{\tan\left( \phi_{p} \right)} \times \left\lbrack {1 + \frac{{\sin\left( \gamma_{p} \right)} + {\sin\left( {\alpha_{p} - \beta_{p} - \gamma_{p}} \right)}}{{\cos\left( \alpha_{p} \right)}\left( {1 + {{\sin\left( {2\phi_{p}} \right)}/{\sin\left( \alpha_{p} \right)}}} \right.}} \right\rbrack}} & {{Equation}2}\end{matrix}$

where γ_(g)=2θ_(p)+α_(p)−π/2. For both scanner designs, both Cyl/Gal(e.g., FIGS. 1A-1C) and Cyl/Poly (e.g., FIGS. 1D-1H), it may bebeneficial that divergence of the beam not change much over the delayrange of the apparatus, and that the beam size remains small compared tothe area of the reflective surfaces. The former calls for a small beamdivergence and long Rayleigh range (z_(R)), thus large beam size,opposing the latter requirement. Generally, one can find designparameters that will satisfy both constraints, and the desired delayrange largely sets the parameters for these competing needs.

Simulation Results for Cyl/Gal and Cyl/Poly

Based on simulations, it is shown that a 50 mm focal length cylindricallens and 2 mm spot diameter are appropriate for a 10 ps delay range witha central wavelength of 1080 nm. The total path length for both Cyl/Galand Cyl Poly mirror arrangements was 100±10 mm, much shorter thanz_(R)=0.75 m, yielding no significant delay-dependent divergence.

FIG. 2 shows wavefront simulations at delays of 0 and −1 mm (−3.3 psround trip) for outputs of the two designs displayed in FIGS. 1A-1H,Cyl/Gal and Cyl/Poly, and for a galvanometer-based delay line using aspherical mirror (Sph/Gal). The Sph/Gal design, as described above, is aprior art design that uses a spherical mirror that lacks the line ofsymmetry introduced herein with the cylindrical mirror. Opticalcomponent positions and angles were optimized to minimize wavefrontaberrations for all simulations. The optimized delay line parameters aregiven in Table 1.

TABLE 1 h R_(p) ϕ θ₀ α β (mm) (mm) Cyl/Gal 70 0 −20 0 8 Cyl/Poly 70 0−90 −80 50.8 Sph/Gal 90 20 20 0 8

Wavefronts calculated for each of the three scanners at zero delay showsome astigmatism, i.e., they have different wavefront curvature alongthe x and y axes. For the Cyl/Gal and Cyl/Poly layouts shown in FIGS.1A-1H, delay-dependent astigmatism is negligible, and the static (zerodelay) astigmatism dominates, even though it is also quite small(<0.2λ). By contrast, both static and delay-dependent astigmatism issignificant for the Sph/Gal design. Here, the astigmatism arises fromarrangement of the spherical mirror. Because ϕ≠0, the beam impinging onthe spherical mirror neither originates from, nor is focused to, thefocal point of the spherical mirror. Astigmatism across the beam rangesfrom ≈λ at zero delay to ≈2.5λ at −1 mm delay, changing sign, axis, andamplitude.

Wavefront tilt (asymmetry in the profiles around the x=0 or y=0 point)varies significantly between scanner designs. Cyl/Gal and Cyl/Poly shownegligible tilt at zero delay, and between them only Cyl/Gal has achange in tilt with delay. For the Sph/Gal design, tilt changessignificantly with delay. At −1 mm (delay=−3.3 ps round trip), tilt haschanged by 2.5λ over the beam diameter. The wavefront plots clearlysuggest that utilizing a focusing element 108 (e.g., cylindrical mirror)having a symmetry line along which off-axis beam deviations can bedirected facilitates dramatically improved wavefront profiles at bothdelay positions shown. In fact, compared to the Sph/Gal design,reduction of beam displacement by ≥10λ and tilt by ≥1000λ is achievedfor the designs introduced in this disclosure. The improvement can becontributed at least in part to the fact that the cylindrical mirror hasa focal axis 112 instead of a focal point like the spherical mirror.Both the movable mirror 106 a,b and the return mirror 110 can be placedon this focal axis 112, ensuring minimal geometric aberrations.

Experimental Results for Cylindrical Design

To demonstrate the utility of the scanner designs described above, theCyl/Gal (and, for reference, the Sph/Gal) scanner was embedded into anFT-CARS arrangement. In each case, 40 fs pulses from a Coherent Fidelity(λ=1073 nm, repetition rate=73 MHz) were split, with one arm passingthrough the delay scanner. A 20 Hz ramp wave was used to drive agalvanometer to ±5 deg. The delayed and test reference arms wereco-linearly recombined, and 1.5 nJ pulses from each were focused to w₀=3μm into CS₂. The first pulse generates a vibrational coherence. Thesecond pulse also generates a coherence, which interferes with the firstat varying time delay. The degenerate four-wave mixing (DFWM) signal ofthe second pulse then generates an interferogram over the staticbackground from the DFWM of the first pulse. The anti-Stokes DFWMsignals were isolated using a 1010 nm shortpass filter and were detectedusing a photodiode as a function of scanner delay. The signal waslow-pass filtered and sampled at 2 MHz, giving 10⁵ points perinterferogram. The laser bandwidth was just sufficient to excite the 655cm⁻¹ v₁ symmetric stretch of CS₂, which has a natural linewidth of <1cm⁻¹.

FIG. 3 depicts the results of testing a Cyl/Gal delay line versus aSph/Gal delay line. The top panel depicts late time section of FT-CARStime trace from the two scanners; the top left inset depicts FT-CARStime trace recording for Cyl/Gal; and the top right inset depicts lighttransmission through a 10 μm pinhole as a function of delay afterpassing through a Cyl/Gal or Sph/Gal scanner. The bottom panel depicts655 cm⁻¹ v₁ symmetric stretch of CS₂ showing from Cyl/Gal delay andSph/Gal scanners.

The left-side inset to the top panel of FIG. 3 shows a DC-subtractedFT-CARS signal transient obtained using the Cyl/Gal scanner. Thetime-zero signal extends to an amplitude of 20 on this scale but istruncated for clarity. The main part of the top panel displays a portionof the FT-CARS signal intensities at delay times ≈5.7 ps, using theCyl/Gal and Sph/Gal scanners, showing that the Cyl/Gal signal fringeoscillations are about 2× stronger at this delay time. Simulations andexperiment suggest that the smaller signal from the Sph/Gal scannercould be due to strongly delay-dependent wavefront tilt leading to poorbeam overlap at long delays. To evaluate delay-dependent beam overlap,the beam exiting the scanner at zero-delay was focused to 10 μm with a50 mm focal length lens. A 10 μm pinhole was placed at the focus, andlight throughput as a function of scanner delay was measured. The rightinset to the top panel in FIG. 3 shows beam transmission for the Sph/Galscanner to be roughly 50% that of the Cyl/Gal scanner at 5.7 ps delay,accounting for the observed drop in FT-CARS signal.

The lower panel of FIG. 3 shows Raman spectra retrieved from Fouriertransforms of FT-CARS interferograms generated in CS₂ using Cyl/Gal andSph/Gal delay lines. Both interferograms were carried to delay timeslong enough to achieve resolution of ≈3.3 cm⁻¹ (delay 10 ps). Therecovered {peak widths, signal-to-noise ratios} for the v₁ peak were{3.5 cm⁻¹, >200} and {33 cm⁻¹, <20} from the Cyl/Gal and Sph/Galdesigns, respectively.

Discussion Related to Delay Lines

Both the Cyl/Gal and Cyl/Poly designs provide for dispersion-free delaylines with static and delay-dependent wavefront aberrations that aresufficiently small to allow transform-limited performance over a delayrange of at least 10 ps, supporting a spectral resolution of ≈3.5 cm⁻¹for FT-CARS and other techniques. This is the first time adispersion-free high-speed-compatible delay scanner has demonstratedtransform-limited performance over such a large scan range. Wavefrontaberrations are minimized in these designs by inducing a symmetry line(here, with the cylindrical mirror). Off-axis beam displacementsassociated with varying time delays can be directed along this symmetryline.

The delay lines described above can be utilized in various opticaltechniques, including interferometry, spectroscopy, and/or coherentanti-Stokes Raman scattering. In the case of Raman microscopy, most ofthe biologically relevant information in a Raman spectrum is found inthe fingerprint where peaks are congested narrow (<5 cm⁻¹ width), makingthem unresolvable with the highspeed mechanical delay lines of the priorart.

Implementations of Delay Line Designs into dFT-BCARS

As described herein, many linear and non-linear optical methods requireoptical delay of one light path with respect to the other.Fourier-transform coherent anti-Stokes Raman scattering (FT-CARS) is oneof these techniques. The technique utilizes two fields (i.e., beams oflight or electric fields) that interact with an object of interest, forexample a biological specimen. One of the two fields is delayed withrespect to the other. To this end, techniques to generate a Ramanspectrum using FT-CARS can utilize the delay line architecture describedabove.

The following describes a new type of technique to generate and detectcoherent Raman spectra. Hereinafter, the new technique can be referredto as dispersed Fourier Transform broadband coherent anti-Stokes Ramanscattering (“dFT-BCARS” or “dispersive FT-BCARS”). The technique canutilize the delay line designs discussed above, or can be used withprior delay line designs. Dispersive FT-BCARS has two points of interestwhich together result in a roughly 100-fold increase in signal levels.One novel aspect of the design is the addition of a spectrally narrowread pulse. The other novel aspect is the application of a detectionscheme wherein generated signal light is spectrally dispersed andfocused on a single-element or multiple-element detector and theresulting electronic signal is summed on the detector.

Background on Anti-Stokes Raman Scattering

When implemented in an imaging modality, Raman spectroscopy providescontrast for many species that otherwise cannot be visualized reliablyor at all in microscopy. It is especially useful for characterizingcomplex systems. In a chemically diverse system such as a biologicalcell, approximately 50 Raman peaks can easily be resolved. With asignal-to-noise ratio (SNR) of 3 in each peak, more than 10²³ differentspectral variations or “colors” can be distinguished. Such rich spectraprovide a basis for highly specific identification of chemical speciesand has application in many fields including clinical and life sciences,corrosion, pharmaceuticals, microelectronics, batteries, carbon-basedmaterials, catalysis, forensics, geology, and mineralogy. While itsnearly universal provision of contrast is a benefit, interpretation ofcomplex spectral can be a challenge, especially if many chemicalcomponents of the sample are unknown, as is frequently the case. Here,data dimension reduction approaches such as multivariate analysis (MVA)approaches or neural networks are typically used to interpret thespectra. These methods connect input data (typically peak amplitude andposition) with known properties of materials connected with the spectra,so are no more reliable than the input spectra. Of course, whether ornot these advanced data mining tools are used, or even if the chemicalcomponents are known, the degree to which one can trust peak shiftpositions and amplitudes determines one's ability to confidently drawconclusions from Raman scattering data.

Precise and reproducible Raman shift frequencies and amplitudes are achallenge for spontaneous Raman spectroscopy. Raman peak amplitudes arereported only qualitatively, and it is not uncommon to see peak shiftvariability on the order of ±5 cm⁻¹ or more for the same vibrationalRaman lines. Perhaps an important reason for this is that, fordispersive (most commonly used) systems, optical transfer functions andwavelength calibration can vary with small alignment changes. In anycase, this imprecision in amplitude and wavelength can be a problem inchemically complex samples, and particularly in a Raman imaging systemwhere slight refractive variation in a sample can change beam alignment.Such shifts are often misinterpreted by MVA approaches as arising fromnew chemical species. Variability on this level is thus likely to be animportant issue as Raman scattering is used increasingly in analysis ofcomplex samples and even in medical diagnosis.

Another important factor is that spontaneous Raman scattering is a veryinefficient process, with only one in 10⁹ photons scatteringinelastically. Although not a problem for samples that can withstandhigh laser flux, acquiring a high-quality Raman spectrum can takeseconds or minutes for most samples and high resolution spontaneousRaman imaging of even a single biological cell can require more than 10hours in state-of-the-art instrumentation. This is thus far too slow tobe applied broadly to such delicate samples. Spatial multiplexing canreduce the average time required for spectral acquisition, butapplications such as food or pharmaceutical product quality control thatmight require rapid imaging are still out of reach, even though routineRaman spectral imaging would be of inestimable value for these.

To more clearly explain the issues at hand, it is beneficial to brieflydescribe generation of an overall broadband coherent anti-Stokes Ramanscattering (BCARS) signal and extraction of the component proportionalto the spontaneous Raman spectrum. In coherent anti-Stokes Raman (CARS),pump and Stokes laser fields interact with a medium to create avibrational coherence or “stimulation profile,” given as:

C _(st)(Ω)=χ⁽³⁾(Ω)[E ^(S) *E ^(p)](Ω)   Equation 3

where E_(p), E_(S) are the frequency-domain pump, and Stokes fields,respectively and * is the cross-correlation operator. Impulsiveexcitation can be achieved if a single ultra-short laser pulsecontaining sufficient bandwidth is used as a pump and Stokes field. Thematerial response to the pump and Stokes laser fields is given by thethird order nonlinear susceptibility, χ⁽³⁾, which contains Ramanvibrationally resonant and non-resonant contributions:

$\begin{matrix}\begin{matrix}{{\chi^{(3)}(\Omega)} = {{\chi_{NR}^{(3)}(\Omega)} + {\chi_{R}^{(3)}(\Omega)}}} \\{= {{\chi_{NR}^{(3)}(\Omega)} + {\sum\limits_{m}\frac{A_{m}}{\Omega_{m} - \Omega - {i\Gamma_{m}}}}}}\end{matrix} & {{Equation}4}\end{matrix}$

where A_(m), Ω_(m), and Γ_(m) describe the amplitude, center frequency,and half-width of the m^(th) Raman peak. χ_(R) ⁽³⁾ is related to thespontaneous Raman spectrum I_(Raman) as I_(Raman)(Ω)∝Im{χ_(R)(Ω)⁽³⁾}.Although χ_(R)(Ω)⁽³⁾ is the quantity of interest in BCARS, the signal isgenerated by interaction of a probe field (E_(pr) (ω)) with thevibrational coherence C_(st)(Ω), so contains contributions from χ⁽³⁾)(Ω)and χ_(NR) ⁽³⁾(Ω)

I _(CARS)(ω+Ω)∝| C _(st)(Ω)*E _(pr)(Ω)|²  Equation 5

∝|χ_(NR) ⁽³⁾(Ω)|²+|χ_(R) ⁽³⁾(Ω)|²+2χ_(NR) ⁽³⁾(Ω)Re[χ_(R)⁽³⁾(Ω)]  Equation 6

where * is the convolution operation, ω is the laser frequency, and Ω isthe Raman shift frequency.

The leftmost term in Equation 6 arises purely from the non-resonantresponse of the medium, and is referred to as the “non-resonantbackground” (NRB). For weak Raman peaks in the fingerprint spectralregion, the NRB can be many orders of magnitude larger than the middleterm containing only χ_(R) ⁽³⁾, making the latter inaccessible. However,because the resonant and non-resonant signals are phase-locked, therightmost term represents a stable heterodyne signal of intermediatestrength containing resonant and non-resonant components. It is thisterm that facilitates detection of weak signals such as fingerprintoscillations from dilute scatterers in biological systems.

To analyze the system, it can be helpful to consider a simplifiedmaterial system with a molecular response (R (t)) consisting of only anon-resonant and a single resonant response:

R(t)=χ_(NR) ⁽³⁾δ(t)+χ_(R) ⁽³⁾Θ_(H)(t)e ^(iΩ) ^(R) ^(t) e ^(−Γ) ^(R)^(t)   Equation 7

where Θ_(H)(t) is the Heaviside function. For the sake of simplicity, itcan also be assumed that the fields E_(p) and E_(S) in Equation 3 botharise from the same pulse, which can be called the “comb” pulse. Inpractice this pulse is ultra-short, typically ≈15 fs in duration, andcan be approximated as delta functions in time. Under this assumption,(R(t)) can be replaced with C_(st)(t) and χ_(NR) ⁽³⁾, χ_(R) ⁽³⁾ withC_(NR), C_(R), which are the amplitudes of the nonresonant and resonantcoherences respectively. They are proportional to the product χ⁽³⁾I₁ ²where I₁ is the comb pulse intensity. Accordingly, the third orderpolarization (P⁽³⁾(t)) in the sample, which leads to the signal field(E_(sig)(t)=iP^((e))(t)), is given as:

P ⁽³⁾(t)=∫_(−∞) ^(∞) C _(st)(τ)E _(pr)(t−τ)dτ   Equation 8

where E_(pr) (t) is the probe field.

FT-CARS as Presently Practiced and Issues with the System

The Fourier transform CARS signal generation and detection process isdiagrammed in FIGS. 4A and 4B. FIG. 4A is a Schematic of FT-CARS asheretofore implemented. Vibrational coherence is induced by each of thetime-delayed laser pulses. Interference between the coherences varieswith their temporal separation Δt₁₂, and this is encoded in a degeneratefour-wave mixing (DFWM) signal generated by comb 2. The two combs areorthogonally polarized to avoid a coherent spike, but this leads to 10Xsignal reduction. In FIG. 4B, the comb profiles are truncated at shortwavelength to provide a spectral region for DFWM anti-Stokes signalcollection.

In FT-CARS, two ultra-short laser comb pulses impinge on the sample witha variable time spacing t₁₂. The first comb pulse provides the pump andStokes fields, exciting a vibrational coherence, C _(st) (n) over thefingerprint spectral region as described in Equation 3; the second combpulse provides E_(p)r and reads the coherence, as described by Equations5 and 8. By design, the first pulse is temporally short enough to excitea significant spectral band (typically up to 2000 cm⁻¹), and the secondpulse is short enough to resolve oscillations of the material vibrationin this same frequency range. Anti-Stokes light scattered from thesecond pulse reflects the temporal oscillations and decay of thematerial coherence generated by the first pulse.

FT-CARS has yielded high-resolution Raman spectra of bulk liquids withvery high signal-to-noise (SNR) in only 14 μs. It holds great appeal formany reasons. These include: 1) signal detection is achieved with amm-sized single element photodiode or PMT, so maintaining alignment istrivial; 2) these detectors have fast response times and direct readout,enabling even GHz signal acquisition rates; 3) the full-time trace isacquired very quickly, reducing the noise-equivalent bandwidth of themeasurement; 4) very high-resolution spectra can be acquired by longerdelays without the intrinsic resolution constraints imposed byspectroscopic instrumentation; 5) Raman shift frequencies can bedetermined to essentially arbitrary accuracy through interferometriccalibration of the second pulse delay; and 6) transforming the collectedinterferograms to spectral domain is accomplished with a single Fouriertransform. A significant drawback to FT-CARS, however, is thatgenerating signal in the higher fingerprint region and the CH-stretch isvery difficult owing to dispersion issues associated with the sub-10 fspulses required to impulsively excite these spectral regions.

A time-domain expression for the FT-CARS signal can be obtained byassuming that pulses 1 and 2 can be represented by temporal deltafunctions. Taking E₁=E₂=1, and t₁=0, then for t₂=Δt>0:

I _(FT)(Δt)∝∫_(−∞) ^(∞)|[C _(ST)(t)+C _(ST)(t−Δt)]E_(pr)[δ(t)+δ(t−Δt)]|² dt  Equation 9

∝2C _(NR) ²+2C _(NR) C _(R)[2+e ^(−ΓΔt) cos(Ω_(R) Δt)]+C _(R) ²[1+2e^(−ΓΔt) cos(Ω_(R) Δt)+½e ^(−2ΓΔt)(cos(Ω_(R) ² Δt)+3)]  Equation 10

Assuming, as usual, that C_(NR)>>C_(R), the rightmost term isnegligible, and the heterodyne term is the delay-dependent component ofinterest. The purely nonresonant terms, containing C_(NR) ² arise fromthe pump-Stokes-probe interaction of a single comb and are independentof Δt. The heterodyne term arises from coherent mixing between thenonresonant field from comb 2 and the resonant field from the probeinteraction of comb 2 with the resonant coherence generated by the firstcomb pulse.

Even with the most advanced FT-CARS approaches, rapid signal acquisitionis possible only for systems such as bulk liquids with large λ⁽³⁾ thatcan withstand high laser flux. These fast FT-CARS measurements usedpulse energies on the order of 10 nJ, whereas spectroscopic BCARSsignals require pulse energies roughly 100 times lower, on the order of50 pJ. There are several reasons for the signal level limitations ofFT-CARS compared to BCARS. First, the two laser pulses in FT-CARS arepolarized orthogonal to one another to avoid a “coherent spike” in thesignal. Thus, the interference detected arises from the difference indepolarized tensor elements |χ₁₂₁₂ ⁽³⁾−χ₁₁₂₂ ⁽³⁾|² (the quantitydetected) is at least 10 times smaller than |χ₁₁₁₁ ⁽³⁾|² detected inBCARS. Second, the ≈15 fs laser pulses used in FT-CARS generate 100 foldhigher NRB levels than the picosecond pulses typically used to read outspectroscopic CARS signals. A too-large NRB limits sensitivity, as itsshot noise will obfuscate even the heterodyne term of Equations 6 or 10for low concentration or weak scatterers. Third, the generated signal isdetected with relatively low efficiency. FIG. 4B shows a spectral domainrepresentation of the excitation pulse and signal profiles. Pulses ofthe same spectral content are used to generate and read the CARS signal,and only the anti-Stokes light of a cutoff filter is detected. Most ofthe generated signal is buried under the excitation profile, and <20% ofthe signal light detected.

The signal-to-background ration of FT-CARS relative to that of BCARS canbe estimated as

where the tilde indicates NRB or signal relative to that of BCARS. Basedon the three factors cited above (e.g., 100-fold higher NRB, 3-foldreduction in coherence generated, and 20% signal collection efficiency),the relative signal to background ratio can be estimated as(0.10*0.2/100)^(1/2)≈0.02, suggesting FT-CARS will have a roughly50-fold worse sensitivity than BCARS, as suggested by experimentalresults discussed above.

New Dispersive FT-BCARS Architecture

Impulsive stimulation architectures can generate high quality CARSfingerprint spectra from low concentration weak scatterers in biologicalsamples. Attention should be paid signal generation and collectionefficiency. In the instant dFT-BCARS implementation, all threesensitivity-reducing issues described above for FT-CARS can be resolvedby introducing a probe pulse of narrow bandwidth (≈3 cm⁻¹) that isblue-shifted from the two pump/Stokes excitation pulses, and temporallysynchronized with one of them, then detecting the signal in a spectrallydispersed way. Hereafter, the newly introduced ps pulse can be referredto as the picosecond or “ps” pulse. The arrangement of dFT-BCARS has thesensitivity already demonstrated in spectroscopic BCARS, but with thesimplified, high-speed signal acquisition characteristic of FT-CARS.

FIG. 5 shows exemplary components of a dFT-BCARS setup. In FIG. 5, alaser source can emit three synchronized laser pulses. Two femtosecondcomb pulses 502 a,b with variable intra-pulse timing (St) are shown. Athird, picosecond pulse 504 is shown that is fixed in time with thefirst femtosecond pulse 502 a. SPD1 and SPD2 are short-pass dichroicfilters that reflect the exciting light, but transmit the signal light.FO1 and FO2 are exciting light and signal light focusing optics used forfocusing the exciting light on the sample and collecting the signallight. FO3 and FO4 are focusing optics that focus the signal light ontodetectors D1 and D2 respectively. DO1 and DO2 are dispersive optics thatspectrally disperse the signal light so that it focuses at the detectorin a line with different colors separated. In the following sections,examples are provided for both generating a signal for the presentdFT-BCARS and detecting the signal from the dFT-BCARS setup.

dFT-BCARS Signal Generation

FIGS. 6A and 6B provide schematic descriptions of the novel dFT-BCARSsignal generation scheme. FIG. 6A shows temporal arrangement of twoultra-short comb pulses that are combined as in FT-CARS, but withparallel (rather than perpendicular) polarization. The dashed linesrepresent comb pulses 1 and 2. The solid lines represent thetime-dependent coherences generated by these pulses. The dash-dot lineshows the field envelope of the third pulse, a temporally extended pspulse that acts as the probe. Because it is several picoseconds induration, it is present during both comb pulses over the entire range ofΔt values applied.

FIG. 6B shows spectral placement of the comb pulses (centered at 200THz) and the probe pulse (centered at 320 THz). The two comb pulsesgenerate degenerate a four-wave mixing (DFWM) signal that is detected inthe usual FT-CARS approach. It is to avoid very strong coherent spikefluctuations in this signal that comb polarizations are setperpendicular in FT-CARS. However, the anti-Stokes signal of interestfor FT-BCARS is shifted away from this spectral region, so is imperviousto the coherent spike, allowing parallel polarization of the combs andproviding 10× higher signal generation efficiency. The “resonant signalcollected” in FIG. 6B represents the anti-Stokes signal of interest inFT-BCARS. Here, as in spectroscopic BCARS, the anti-Stokes light isgenerated by two distinct mechanisms and covers both the fingerprint andCH-stretch spectral regions. The two coherence-generating mechanisms arereferred to “impulsive” and “2-color.” The former is also operative inFT-CARS, where coherences in the fingerprint spectral region aregenerated with pump and Stokes fields from a single pulse. Here, howeverunlike FT-CARS, a ps pulse provides the probe field. In the 2-colormechanism, the ps pulse provides the pump and probe fields, and the combprovides only the Stokes field. Together, these to mechanisms broadenthe range of coherences C_(st,1) and C_(st,2) into the CH-stretchregion, and the signal over the spectral range (3 to 3000) cm⁻¹ can beobtained without the need for sub-10 fs pulses.

The time-domain expression for the FT-BCARS signal is similar to that ofFT-CARS, but the two delta-function probe pulses can be replaced with asingle-frequency probe field:

$\begin{matrix}{{I_{{FT} - B}\left( {\Delta t} \right)} \propto {\int_{- \infty}^{\infty}{{❘{\left\lbrack {{C_{ST}(t)} + {C_{ST}\left( {t - {\Delta t}} \right)}} \right\rbrack E_{pr}}❘}^{2}{dt}}}} & {{Equation}11}\end{matrix}$ $\begin{matrix}{\propto {{2C_{NR}^{2}} + {2C_{NR}{C_{R}\left\lbrack {2 + {e^{r\Delta t}{\cos\left( {\Omega_{R}\Delta t} \right)}}} \right\rbrack}} + {C_{R}^{2}\frac{\left. {{\left( {{2\Gamma^{2}} + \Omega^{2}} \right)\left( {1 + {e^{{- r}\Delta t}{\cos\left( {\Omega_{R}\Delta t} \right)}}} \right)} - {{\Omega\Gamma}e^{{- r}\Delta t}{\sin\left( {\Omega_{R}\Delta t} \right)}}} \right)}{2{\Gamma\left( {\Gamma^{2} + \Omega^{2}} \right)}}}}} & {{Equation}12}\end{matrix}$

Recognizing that typical values for Γ and Ω are 0.3 THz and 30 THzrespectively, the approximate substitution (2Γ²+Ω²)≈(Gamma²+Ω²)≈Ω² inthe purely resonant contribution can be made, reducing Equation 12 to:

I _(FT-B)(Δt)∝2C _(NR) ²+2C _(NR) C _(R)[2+e ^(−ΓΔt) cos(Ω_(R) Δt)]+C_(R) ²[1+e ^(−ΓΔt) cos(Ω_(R) Δt)]/2Γ   Equation 13

where it can be noted that the purely resonant term differs from theother terms in that it is divided by frequency (or, equivalently,multiplied by time.) This factor represents the effective time overwhich the signal is generated and collected. It is not explicit in thefirst two terms because they involve temporal delta-functions.

The expressions for FT-CARS and FT-BCARS in Equations 10 and 13 haveimportant similarities and differences. The purely non-resonant andheterodyne terms have identical form in FT-CARS and FT-BCARS. Thetemporally long probe has little effect on the magnitude of thenon-resonant signal because the non-resonant material coherence persistsonly for the duration of the short comb pulses (approximated bydelta-functions here). By contrast, the purely resonant term differsbetween the two expressions in that it is divided by 2Γ in the FT-BCARSscheme. This reflects the duration over which the resonant signal iscollected and is to be compared to the Γ value for the comb pulses,which is implicitly included in the other terms. As mentioned above, thecombs have a duration of ≈15 fs (so, Γ≈67 THz), which is roughly 100times larger than Γ characteristic of vibrational resonances thatappears in the last term of Equation 13. Thus, a 100-fold increase maybe expected in the purely resonant signal in FT-BCARS compared toFT-CARS.

As with FT-CARS, the Δt-dependent anti-Stokes signal amplitude ofFT-BCARS is simply Fourier transformed to yield the Raman spectrum.Here, however, the signal covers the (3 to 3000) cm⁻¹, range rather thanjust the fingerprint. Adding the picosecond probe pulse may intensifythe purely resonant signal, but this will still typically be muchsmaller than the heterodyne contribution, and when it is not, the signalwill transition from a linear concentration dependence (due to theheterodyne term) to a quadratic concentration dependence due to thepurely resonant term.

dFT-BCARS Signal Detection

To the extent that the non-resonant material response is instantaneous,the non-resonant signal created by ultra-short comb-probe interactionshas the same ultra-short (roughly 15 fs) duration at the detector, andthus can only heterodyne amplify the first 15 fs of the resonant signalgenerated during the same comb pulse interaction. In this case, thesystem fails to heterodyne amplify any of the increased signal intensityin the last term of Equation 13, so the increased signal may still bebelow the detection noise.

In spectroscopic BCARS, the non-resonant and resonant signals arestretched and overlapped in time when they are spectrally dispersed onto the CCD camera. In that approach, the heterodyne interaction lastsfor the full duration of the material coherence, so all of the collectedresonant signal is amplified. In this case, the heterodyne signaldominates, providing linear concentration dependence and highsensitivity.

As in spectroscopic BCARS, in FT-BCARS, the system can cause temporallydistinct signal components to interact by spectrally dispersing on amulti-element detector and thus temporally stretching them. However,with FT-BCARS, the system is not constrained to use a multi-elementdetector as in spectroscopic BCARS because the FT-BCARS signal isencoded in the time domain. Instead, a single-element or few-elementdetector can be used.

The temporal behavior of distinct spectral signal components can beconsidered. The analysis can start by finding an expression for C_(st)(Ω). This can be obtained as the Fourier transform of R(t)described in Equation 7, multiplied by the exciting field and convolvedwith the probe field in the frequency domain:

$\begin{matrix}\begin{matrix}{{{\overset{\sim}{P}}_{n}\left( {\omega;t_{n}} \right)} = {\mathcal{F}\left\{ {{\chi_{NR}^{(3)}{\delta\left( {t - t_{n}} \right)}} + {\chi_{R}^{(3)}{\Theta_{H}\left( {t - t_{n}} \right)}e^{i{\Omega_{R}({t - t_{n}})}}e^{- {\Gamma_{R}({t - t_{n}})}}}} \right\}}} \\{\left\lbrack {E_{S}{\bigstar E}_{P}} \right\rbrack(\Omega)*{E_{pr}(\omega)}} \\{{= {\frac{\sigma_{\omega}}{2}{{Exp}\left\lbrack {{- \left( {\omega/\left( {\sqrt{2}\sigma_{\omega}} \right)} \right)^{2}} + {{it}_{n}\omega}} \right\rbrack}}}\left\lbrack {C_{NR} + \frac{C_{R}}{\left( {\Gamma_{R} - {i\left( {\omega + \Omega_{R}} \right)}} \right)}} \right\rbrack}\end{matrix} & {{Equation}14}\end{matrix}$

where σ_(ω) characterizes the spectral width of and t_(n) gives thearrival time of the n^(th) comb pulse that provides the pump and Stokesfields for {tilde over (P)}_(n).

If the signal is not dispersed on the detector, the temporal duration ofnonresonant signal component is given by Exp[−t²/2σ_(ω) ²]. For σ_(ω)=25THz, the nonresonant signal field persists for about 15 fs. As discussedabove, the resonant component will persist much longer because it isemitted continuously over the period of the probe pulse. If the pulse isspectrally dispersed, the nonresonant field component of each individualspot will have a temporal duration extended by roughly a factor N, thenumber of spectrally distinct spots created. In the time domain,expressions for the spectrally dispersed nonresonant and resonant fieldcontributions at the detector are given as:

$\begin{matrix}{{E_{NR}\left( {t;k} \right)} \propto {\frac{C_{NR}\sigma_{\omega}}{\sqrt{2}N}{\cos\left( {\omega_{k}t} \right)}\left\{ {{\delta(t)}*\exp^{\lbrack{- {({\sigma_{\omega}{t/2}N})}^{2}}\rbrack}} \right\}}} & {{Equation}15}\end{matrix}$ $\begin{matrix}{{E_{R}\left( {t;k} \right)} \propto {\frac{C_{R}\sigma_{\omega}}{\sqrt{2}}\exp^{({i\omega_{k}l})}\left\{ {{\Theta_{H}(t)}\exp^{{- \Gamma_{k}}l}*\exp^{\lbrack{- {({\sigma_{\omega}{t/2}N})}^{2}}\rbrack}} \right\}}} & {{Equation}16}\end{matrix}$

where a factor N is present in the denominator of the expression forE_(NR) but missing from the denominator of E_(R) since C_(R) is in itsdefinition summed only over the specific resonance frequency, whereasC_(NR) is defined as summed over all frequencies. Under this detectionscheme, an electrical signal proportional to the modulus squared of theelectric field at each spot is generated and these are all summed at thedetector.

$\begin{matrix}{{Sig} \propto {\sum\limits_{k}^{N}{\int_{- \infty}^{\infty}{{❘{{E_{1}\left( {t;k} \right)} + {E_{2}\left( {{t - {\Delta t}},h} \right)}}❘}^{2}{dt}}}}} & {{Equation}17}\end{matrix}$

FIG. 7 shows results of numerical integration of Equation 17 for N=1(non-dispersed FT-BCARS) and N=1000 (dispersed FT-BCARS), as a functionof the ratio χ_(R) ⁽³⁾/χ_(NR) ⁽³⁾. The results for FT-CARS are alsoshown in the dashed line. In FT-CARS, the heterodyne and purely resonantterms have similar functional terms (see Equation 10). Thus, when χ_(NR)⁽³⁾>>χ⁽³⁾, the heterodyne term dominates, and the signal is linear inλ_(R) ⁽³⁾. At larger values of χ_(R) ⁽³⁾, the purely resonant termdominates, and the signal turns over to quadratic. For fingerprintspectra it is almost never the case that χ_(R) ⁽³⁾>χ_(NR) ⁽³⁾, and forall practical purposes, the signal can always be considered linear.

In the FT-BCARS scheme a blue-shifted ps laser pulse acts as the probe.This allows use of parallel-polarized comb pulses and to collect a muchlarger fraction of the anti-Stokes light. Together these factors lead toa 40-fold signal increase, however this increase will generally beoffset by a limit in the amount of light that can be used safely on thesample, and the fact that that light will be distributed between threepulses instead of two. Overall, the redistribution of light may lead toa reduction in signal by a factor of 2 to 10.

Example Use Cases

The following section provides example use cases for implementing thedelay line architecture and optical techniques discussed above. Thefollowing examples are not limiting, but instead provide illustrationsof how the systems and methods may be used and/or combined for opticaltechniques.

Referring again to FIGS. 1C and 1H, both examples show a delay module100 for creating a delay line. A source 104 can provide an electricfield such a light field 102. The light field 102 can be directed to amovable mirror 106 a,b. Depending on the implementation, the movablemirror can be, for example, a planar galvanometer mirror (e.g., movablemirror 106 a in FIG. 1C) or a polygonal mirror (e.g., movable mirror 106b in FIG. 1H). The movable mirror 106 a,b can be rotatable upon an axis.In the case of the galvanometer movable mirror 106 a, the rotation canbe about an axis 107 running along the center of the movable mirror 106a such that the movable mirror 106 a can rotate, or oscillate, side toside. In the case of the polygonal movable mirror 106 b, the rotationcan be about a rotational axis 116 running through a center point of themovable mirror 106 b such that the movable mirror 106 b can rotate, orspin, about the rotational axis 116.

In either case, e.g., the planar galvanometer mirror 106 a or thepolygonal mirror 106 b, the direction of the incoming light field 102can be redirected to the focusing element 108. The focusing element 108be a single cylindrical mirror, as shown in FIGS. 1C and 1H, or it canbe a plurality of cylindrical mirrors. The one or more cylindricalmirrors can, because of the one-dimensional concavity of theirstructure, have a focal axis 112 parallel to the length of the focusingelement 108. This, of course, is a different structure than found in thespherical mirrors of the prior art which have two-dimensional concavityand a point focus rather than a focal axis. Although the focal axis 112is shown placed upon the focusing element 108 in FIGS. 1C and 1H, itwill be understood that the actual focus of the focusing element 108 iscloser to the incoming light field 102. This enables the light field 102to be received by the focusing element 108 and reflected back to areturn mirror 110 along the focal axis 112. The return mirror 110 can beplaced at the actual focal line of the focusing element 108, so that theline-focused beams reflected along the z-direction by the focusingelement 108 are returned along the optical path.

The light field 102 can then be returned from the return mirror 110, tothe focusing element 108, to the movable mirror 106 a,b, and back to thesource 104. The returned light field is now delayed with respect to theincoming light field 102. In some examples, the process of sending thelight field through the delay module 100 can be repeated a plurality oftimes to extend a delay range and/or reduce aberrations.

FIG. 8A is a schematic for implementing the example delay module 100described above. In the configuration of FIG. 8A, an input field (e.g.,the light field 102 described above) can be sent to a beam splitter. Aportion of the split light field, e.g., the first light field, can besent to the delay module 100. Another portion of the split light field,e.g., the second light field, can be directed to an object of interest.For example, the configuration in FIG. 8A can be used in Lidar, whereinthe object of interest is some distant reference target, in opticalcoherence tomography (OCT), wherein the object of interest includes thefeatures of the retina, etc. After the second light field interacts withthe object of interest, it is returned to the beam splitter, combinedwith the delayed line (which is delayed by the delay module 100), andanalyzed. The present delay module 100 architecture enables rapiddistance measurements (e.g., scan rates of greater than 40.0 kHz) withsub-micron resolution. This can be enabled by the reduction ofaberrations by using the focusing element 108 and by changing thegeometry of the movable mirrors 106 a,b over the prior art designs.

FIG. 8B is a diagram of using the delay module 100 in a signalauto-correlation technique. An original light field can be directed toan object of interest. A first light field is then created after theoriginal light field passes through or is scattered by the object ofinterest, thereby creating a first light field with new spectralfrequency content compared to the original light field. The first lightfield can then be collected and directed to a first beam splitter suchthat the first light field is split into the second light field and athird light field. The second light field then passes through the delaymodule 100, as described above, to create the delayed light field. Thedelayed light field is recombined with the third light field to createcombined light fields. The combined light fields can then be directed toone or more detectors and analyzed. For example, a frequency-dependentamplitude of the combined light fields can be determined based on adelay between the delayed light field and the third light field. Inthese examples, the signal field (i.e., the second light field in thefigure) uses itself as a local oscillator.

The original light field can be an optical pulse with spectral contentof greater than 200 wavenumbers, for example from approximately 200 toapproximately 3500 wavenumbers. The first and third light fields can bedetected at optical frequencies higher than those in the original lightfield, for example in the case of anti-Stokes scattering. In otherexamples, the delayed and third light fields can be detected at opticalfrequencies lower than those in the original light field, for example inthe case of Stokes scattering.

In some examples, the original light field can be a combination of twolight fields, for example a first original light field and a secondoriginal light field. The first original light field and the secondoriginal light field can be combined co-linearly and coincidentally intime prior to passing through the first beam splitter. The firstoriginal light field can be an optical pulse with spectral content ofgreater than 200 wavenumbers; the second original light field can be anoptical pulse with spectral content of less than 30 wavenumbers. Thedelayed and third light fields can be detected at optical frequencieshigher than those in the original light field, for example in the caseof anti-Stokes scattering. In other examples, the delayed and thirdlight fields can be detected at optical frequencies lower than those inthe original light field, for example in the case of Stokes scattering.

In some examples, the combined light fields can be detected along theirprimary polarization direction. The physics of waves is such that, whenlight of the same frequency and polarization is combined into a singleoptical mode, (such as is the case when applying interferometricdetection methods) it may be necessary that the light have at least twoalternative paths along which to propagate after combination. For twoincoming light fields of the same energy, with two fields emerging aftercombination, the two emerging fields vary in energy between zero and thesum of the incoming field energies depending on the phase (i.e., thepath length) difference between the two incoming light fields. The totalenergy in the light propagating along both post-combination paths isequal to the sum of the energies of the two incoming light fields (minusany losses in the combining optics). By measuring the light energy inone post-combination path, it is possible to detect oscillations in thesignal that contain information about the phase and amplitude of lightfrequency components that are common to both incoming fields. Thesesingle-path measurements are convenient in that they can be done withonly one detector. However, amplitude noise that is difficult to rejectcan be introduced by intensity fluctuations in the incoming lightfields.

Another approach is to detect both paths simultaneously, but ondifferent detectors. By doing this, one can compute the differencebetween and sum of the two detector signals and divide the former by thelatter. In doing this, one obtains a normalized difference signal thatis impervious to source light field fluctuations. To this end, thecombined light fields can be separately and simultaneously detected at+45 degrees and at −45 degrees with respect to their primarypolarization direction.

FIG. 8C is a diagram of using the delay module 100 in across-correlation technique. An original light field can be directedacross a first beam splitter such that the original light field is splitinto the first light field and a second light field. The first lightfield then passes through the delay module 100, as described above, tocreate the delayed light field. The second light field can be directedto an object of interest thereby creating a third light field having adifferent spectral phase or frequency content than the second lightfield. The third light field can be collected and combined with thedelayed light field at a second beam splitter, thereby creating combinedlight fields. The combined light fields can be directed onto one or moredetectors and analyzed. The component of the delayed light field that iscombined with the third light field at the second beam splitter isreferred to as the local oscillator. For example, thefrequency-dependent amplitude of the third light field can be determinedbased on a delay between the delayed light field and the third lightfield. In these examples, the local oscillator component of the delayedlight field can originate before or after the first beam splitter.

The original light field can be an optical pulse with spectral contentof greater than 200 wavenumbers, for example from approximately 200 toapproximately 3500 wavenumbers. The third light field can be detected atoptical frequencies higher than those in the original light field, forexample in the case of anti-Stokes scattering. In other examples, thethird light field can be detected at optical frequencies lower thanthose in the original light field, for example in the case of Stokesscattering.

In some examples, the original light field can be a combination of twolight fields, for example a first original light field and a secondoriginal light field. The first original light field and the secondoriginal light field can be combined co-linearly and coincidentally intime prior to passing through the first beam splitter. The firstoriginal light field can be an optical pulse with spectral content ofgreater than 200 wavenumbers; the second original light field can be anoptical pulse with spectral content of less than 30 wavenumbers. Thethird light field can be detected at optical frequencies higher thanthose in the original light field, for example in the case ofanti-Stokes scattering. In other examples, the third light field can bedetected at optical frequencies lower than those in the original lightfield, for example in the case of Stokes scattering.

In some examples, the combined light fields can be detected along theirprimary polarization direction. In other examples, the combined lightfields can be separately and simultaneously detected at +45 degrees andat −45 degrees with respect to their primary polarization direction.

FIG. 8D is a diagram of implementing the example in FIG. 8C, i.e., thecross-correlation technique, with a material 802 in line with thedelayed light field to generate the local oscillator portion of thedelayed light field that has the same frequency components as the thirdlight field (i.e., the field crated by interacting with the object ofinterest). The material 802 can be placed in line with the delayed lightfield such that the frequency of the delayed light field is alteredprior to combining with the third light field. The material 802 wouldpreferentially be a material with a strong inelastic scatteringcross-section or a high optical nonlinearity (e.g., large χ⁽³⁾) whereininteraction with the first light field would generate light at newcolors (i.e. at new frequency components).

In another alternative, instead of the material 802 being placed in linewith the delayed light field, the original light field can contain thesame frequencies as the third light field, and thus the local oscillatorfield. The first beam splitter can be dichroic, thereby separating afrequency of the first light field from a frequency of the originallight field that are common to the third light field (the localoscillator).

FIG. 9A is a schematic for implementing the example delay module 100described above. In the configuration of FIG. 9A, multiple fields caninteract with an object of interest, and at least one of the fields canbe delayed in time with respect to the other. This example configurationcan be used to implement fingerprint coherent Raman microscopy systemsand methods. An input light field (e.g., the light field 102 describedabove) can be sent to a beam splitter. A portion of the split field,e.g., the first light field, can be sent to the delay module 100.Another portion of the split field, e.g., the second light field, can bedirected to a fixed mirror, such that the second field is not delayed.The non-delayed second light field can be combined with the delayedlight field to create combined light fields. The combined light fieldscan be directed across an object of interest, thereby creating one ormore new light fields with spectral frequency content. As describedabove, BCARS and other fingerprint coherent Raman microscopy techniquescan be used to analyze biological samples or material samples. To thisend, the object of interest can be a biological sample or materialsample in these examples. The one or more new light fields created byinteraction with the object of interest can be directed onto one or moredetectors and analyzed. For example, a frequency-dependent phase and/oramplitude of the one or more new light fields can be determined based ona delay between the delayed light field and the second light field.

The first light field and the second light field can be optical pulseswith spectral content of greater than 200 wavenumbers. The one or morenew light fields can be detected at higher optical frequencies than thedelayed light field and the second light field, for example inanti-Stokes scattering. In other examples, the one or more new lightfields can be detected at lower optical frequencies than the delayedlight field and the second light field, for example is Stokesscattering.

FIG. 9B is another example that can be used for fingerprint coherentRaman microscopy. The example shown in FIG. 9B is similar to the exampleshown in FIG. 9A. In this example, however, a third light field 902 canbe applied to the second light field. The third light field 902 can bean optical pulse with spectral content of less than 30 wavenumbers, andcan be fixed in time with the second (non-delayed) light field. Thecombination of the delayed light field, the second light field, and thethird light field 902 can create one or more new light fields afterinteracting with an object of interest, and the one or more new lightfields can then be analyzed.

As described above, the third light field 902 can be a picosecond pulsethat acts as the probe in the dFT-BCARS examples described above. Thispicosecond pulse is shown as picosecond pulse 504 in FIG. 5. The firstlight field and the second light field can be femtosecond pulses, shownin FIG. 5 as first femtosecond pulse 502 a and second femtosecond pulse502 b. When the third light field 902 is several picoseconds induration, the third light field 902 can be present during both thesecond light field pulse and the delayed light field pulse.

It should be noted that, although FIGS. 9A and 9B show example systemsusing the delay module 100 described above, the delayed light field thatis combined with the second light field in these examples can be createdby any other system or method. As will be appreciated, the dFT-BCARSsystems and methods described above can utilize any method of delayingone of the two fields that enter the object of interest, and the utilityof the dFT-BCARS design is not limited to using the delay module 100defined above.

It is to be understood that the embodiments and claims disclosed hereinare not limited in their application to the details of construction andarrangement of the components set forth in the description andillustrated in the drawings. Rather, the description and the drawingsprovide examples of the embodiments envisioned. The embodiments andclaims disclosed herein are further capable of other embodiments and ofbeing practiced and carried out in various ways. Also, it is to beunderstood that the phraseology and terminology employed herein are forthe purposes of description and should not be regarded as limiting theclaims.

Accordingly, those skilled in the art will appreciate that theconception upon which the application and claims are based may bereadily utilized as a basis for the design of other structures, methods,and systems for carrying out the several purposes of the embodiments andclaims presented in this application. It is important, therefore, thatthe claims be regarded as including such equivalent constructions.

Furthermore, the purpose of the foregoing Abstract is to enable theUnited States Patent and Trademark Office and the public generally, andespecially including the practitioners in the art who are not familiarwith patent and legal terms or phraseology, to determine quickly from acursory inspection the nature and essence of the technical disclosure ofthe application. The Abstract is neither intended to define the claimsof the application, nor is it intended to be limiting to the scope ofthe claims in any way. Instead, it is intended that the invention isdefined by the claims appended hereto.

1. A method comprising: directing combined light fields across an object of interest thereby creating one or more new light fields with new spectral frequency content; directing one or more of the new light fields onto one or more detectors after transmission through or scattering from the object of interest; and determining a frequency-dependent phase and/or amplitude of one or more of the new light fields based on a delay between light fields of the combined light fields.
 2. The method of claim 1 further comprising: directing a first light field to a delay module; and delaying the first light field via the delay module to create a delayed light field; wherein the combined light fields comprise the delayed light field and a second light field.
 3. The method of claim 1, wherein: the delay module comprises: a movable mirror; a focusing optical element having a focal axis parallel to its length; and a return mirror; the movable mirror is configured to receive the first light field and reflect the first light field to the focusing optical element; the focusing optical element is configured to receive the first light field reflected from the movable mirror and return the first light field to the return mirror; the return mirror is configured to receive the first light field reflected from the focusing optical element and reflect the first light field back to the focusing optical element.
 4. The method of claim 3, wherein: the movable mirror is a planar galvanometer mirror rotatable upon an axis; and the method further comprises rotating the movable mirror upon the axis to direct the first light field toward the focusing optical element.
 5. The method of claim 3, wherein: the movable mirror is a polygonal mirror rotatable around a rotational axis; and the method further comprises rotating the movable mirror upon the rotational axis to direct the first light field toward the focusing optical element.
 6. The method of claim 2 further comprising repeating the directing and the delaying to extend a delay range and/or reduce aberrations. 7.-8. (canceled)
 9. The method of claim 2 further comprising: directing an input light field across a beam splitter such that the input light field is split into the first light field directed to the delay module and a second light field directed to the object of interest; and calculating a complex refractive index of the object of interest based on optical interference between the delayed light field and the second light field after the second light field interacts with the object of interest.
 10. The method of claim 2 further comprising: directing an input light field across a beam splitter such that the input light field is split into the first light field and the second light field; and combining the second light field and the delayed light field to create the combined light fields.
 11. The method of claim 10, wherein: the first light field and the second light field are optical pulses with spectral content of greater than 200 wavenumbers; and either the one or more new light fields are detected at higher optical frequencies than the delayed light field and the second light field; or the one or more new light fields are detected at lower optical frequencies than the delayed light field and the second light field.
 12. The method of claim 10 further comprising: combining another light field containing the same frequency components as the one or more new light fields; and constituting a local oscillator with any of the light fields previous to detection.
 13. The method of claim 10 further comprising: combining a third light field with the combined light fields, the third light field being an optical pulse with spectral content of less than 30 wavenumbers and being fixed in time with the second light field; and directing the combined light fields with the third light field across the object of interest to create the one or more new light fields; wherein either: the one or more new light fields are detected at higher optical frequencies than the third light field; or the one or more new light fields are detected at lower optical frequencies than the third light field.
 14. The method of claim 10 further comprising detecting, at one or more of the detectors, one or more of the new light fields along their primary polarization direction.
 15. The method of claim 10 further comprising detecting one or more of the new light fields at one or more of the detectors; wherein one or more of the new light fields are separately and simultaneously detected at +45 degrees and at −45 degrees with respect to their primary polarization direction.
 16. A method comprising: directing an original light field across a first beam splitter such that the original light field is split into a first light field and a second light field; delaying the first light field to create a delayed light field; directing the second light field across an object of interest thereby creating a third light field having new spectral frequency components compared to the second light field; collecting the third light field after the second light field scatters from the object of interest and/or after the second light field transmits through the object of interest; combining the third light field with a portion of the delayed light field having the same frequency components as the third light field in a second beam splitter, thereby creating combined light fields; directing the combined light fields onto one or more detectors; and determining a frequency-dependent phase and/or amplitude of the combined light fields based on a delay between the delayed light field and the second light field.
 17. The method of claim 16 further comprising directing, prior to combining the third light field with the portion of the delayed light field, the delayed light field into a material to generate the portion of the delayed light field having the same frequency components as the third light field.
 18. The method of claim 16, wherein: the original light field contains the same frequencies as the third light field; and the first beam splitter is dichroic, thereby separating a frequency of the first light field from a frequency of the original light field that are common to new frequency components of the third light field.
 19. The method of claim 16, wherein: the original light field is an optical pulse with spectral content of greater than 200 wavenumbers; and either the third light field is detected at optical frequencies higher than those in the second light field; or third light field is detected at optical frequencies lower than those in the second light field.
 20. The method of claim 16, wherein: the original light field is a combination of a first original light field and a second original light field; the first original light field and the second original light field are combined co-linearly and coincidentally in time; the first original light field is an optical pulse with spectral content of greater than 200 wavenumbers; the second original light field is an optical pulse with spectral content of less than 30 wavenumbers; and either the third light field is detected at optical frequencies higher than those in the second light field; or the third light field is detected at optical frequencies lower than those in the second light field.
 21. The method of claim 16, wherein the combined light fields are detected along their primary polarization direction.
 22. The method of claim 16, wherein: the combined light fields are separately and simultaneously detected at +45 degrees and at −45 degrees with respect to their primary polarization direction.
 23. A method comprising: directing an original light field across an object of interest, thereby creating a second light field with new spectral frequency content compared to the original light field; collecting the second light field after it scatters from the object of interest or after it transmits through the object of interest; directing the second light field across a first beam splitter such that the second light field is split into a first light field and a third light field; delaying the first light field to create a delayed light field; combining the delayed light field and the third light field in a second beam splitter to create combined light fields; directing the combined light fields onto one or more detectors; and determining a frequency-dependent amplitude of the combined light fields based on a delay between the delayed light field and the third light field.
 24. The method of claim 23, wherein the original light field is an optical pulse with spectral content of greater than 200 wavenumbers; and either the first and third light fields are detected at optical frequencies higher than those in the original light field; or the first and third light fields are detected at optical frequencies lower than those in the original light field.
 25. The method of claim 23, wherein: the original light field is a combination of a first original light field and a second original light field; the first original light field and the second original light field are combined co-linearly and coincidentally in time; the first original light field is an optical pulse with spectral content of greater than 200 wavenumbers; the second original light field is an optical pulse with spectral content of less than 30 wavenumbers; and either the first and third light fields are detected at optical frequencies higher than those in the original light field; or the first and third light fields are detected at optical frequencies lower than those in the first light field.
 26. The method of claim 23, wherein the first light field and the third light field are detected along their primary polarization direction.
 27. The method of claim 23, wherein the first light field and the third light field are separately and simultaneously detected at +45 degrees and at −45 degrees with respect to their primary polarization direction.
 28. A system comprising: a light field source configured to provide an input light field; a beam splitter configured to split the input light field into a first light field and a second light field; a delay module configured to delay the first light field to create a delayed light field; a detector configured to combine the second light field with the delayed light field after the second light field interacts with an object of interest; and a processor; wherein the delay module comprises: a movable mirror; a focusing optical element having a focal axis parallel to its length; and a return mirror; wherein the movable mirror is configured to receive the first light field and reflect the first light field to the focusing optical element; wherein the focusing optical element is configured to receive the first light field reflected from the movable mirror and return the first light field to the return mirror; wherein the return mirror is configured to receive the first light field reflected from the focusing optical element and reflect the first light field back to the focusing optical element; and wherein the processor is configured to calculate a complex refractive index of the object of interest based on optical interference between the delayed light field and the second light field after the second light field interacts with the object of interest.
 29. The system of claim 28, wherein: the movable mirror is rotatable upon an axis; the movable mirror is positioned such that the movable mirror intersects the focal axis of the focusing optical element; the return mirror is positioned such that the return mirror intersects the focal axis of the focusing optical element; and the first light field approaches the delay module along a line which is a linear combination of the axis of the movable mirror and the focal axis of the focusing optical element.
 30. The system of claim 28, wherein the focusing optical element is a cylindrical mirror; wherein the movable mirror is selected from the group consisting of a planar galvanometer mirror and a polygonal mirror; and the system has a scan rate of greater than 1.0 kHz. 31.-33. (canceled)
 34. The system of claim 30 having a scan rate of greater than 40.0 kHz.
 35. The method of claim 1 further comprising: directing an input light field across a beam splitter such that the input light field is split into a first light field and a second light field; delaying the first light field with respect to the second light field, thereby creating a delayed light field; and combining a third light field with the second light field and the delayed light field to create combined light fields, the third light field being an optical pulse with spectral content of less than 30 wavenumbers and being fixed in time with the second light field. 36.-38. (canceled)
 39. The method of claim 35, wherein delaying the first light field with respect to the second light field comprises directing the first light field to a delay module, the delay module comprising: a movable mirror; a focusing optical element having a focal axis parallel to its length; and a return mirror; wherein the movable mirror is configured to receive the first light field and reflect the first light field to the focusing optical element; wherein the focusing optical element is configured to receive the first light field reflected from the movable mirror and return the first light field to the return mirror; and wherein the return mirror is configured to receive the first light field reflected from the focusing optical element and reflect the first light field back to the focusing optical element.
 40. The method of claim 39, wherein: the movable mirror is rotatable upon an axis; the movable mirror is positioned such that the movable mirror intersects the focal axis of the focusing optical element; the return mirror is positioned such that the return mirror intersects the focal axis of the focusing optical element; the first light field approaches the delay module along a line which is a linear combination of the axis of the movable mirror and the focal axis of the focusing optical element; and the method further comprises rotating the movable mirror upon the axis to direct the first light field toward the focusing optical element. 41.-44. (canceled)
 45. A method comprising: directing a first light field across an object of interest; directing a delayed light field across the object of interest; directing a picosecond probe light field across the object of interest; creating a signal field with the first light field, the delayed light field, and the picosecond probe light field after the first light field, the delayed light field, and the picosecond probe light field interact with the object of interest; and analyzing the signal field with a broadband coherent anti-Stokes Raman scattering technique after applying delay on the delayed light field.
 46. The method of claim 45 further comprising: creating the delayed light field by: passing a second light field through a beam splitter to create a third light field; and directing the third light field to a movable mirror, to a cylindrical mirror having a focal axis parallel to a length of the cylindrical mirror, and to a return mirror.
 47. The method of claim 45 further comprising: dispersing the signal field before applying it onto a single-element or multi-element detector; varying timing of the delayed light field; and recovering signal oscillations based on a variation of the delayed light field. 